1. Technical Field
This invention relates to wireless subscriber systems and, more particularly, the use of devices for granting or denying access to the wireless subscriber system.
2. Discussion
Some wireless subscriber systems utilize identification codes which are transmitted by wireless subscriber units along with other data sent to a receiver/transmitter site, base station, or cell. Electronics associated with the receiver/transmitter site can identify the wireless subscriber unit by the identification code. The identification codes can be used for billing the wireless subscriber unit for "air time" on a subscriber system or telephone exchange in addition to a basic monthly rate.
Such identification codes can be intercepted during an authorized user's transmission to the subscriber service or telephone exchange. The identification codes can then be programmed into an unauthorized wireless subscriber unit allowing the fraudulent user to gain access to the telephone exchange. The use of the subscriber service or telephone exchange by the unauthorized user is usually incorrectly billed to the authorized user and, of course, the operators of the subscriber system or telephone exchange are typically unable to collect the basic monthly rate from the unauthorized user.
Other wireless subscriber units do not automatically transmit an identification code during transmission. Identification of these wireless subscriber units is desirable when they are interfering with the transmission of other users, when they are being operated in a clandestine manner, or when they are otherwise being misused.
In an effort to address the above problems, radio fingerprinting was developed in the 1940's and 1950's. Oscilloscope photos or hand drawings of amplitude or frequency-detected turn-on transients, turn-off transients, and a final resting frequency of received radio transmitter signals were prepared. The photos or drawings reflect the damping factor and natural period of the radio transmitter and were visually compared to previous measurements to identify the radio transmitter. However radio fingerprinting became more difficult beginning in the 1960's due to an increasing number of radio transmitters and to manufacturing consistency of modern day radio transmitters. Thus, the turn-on transients, turn-off transients, and final resting frequency of the radio transmitters became far less distinguishable from each other using the radio fingerprinting method.
In U.S. Pat. No. 5,005,210 to Ferrell, a turn-on transient of a frequency demodulated waveform from a transmitter is captured and analyzed. Transmitters are identified by measuring signal phase of the turn-on transient or phase response of the turn-on transient with respect to a predetermined frequency. Commercial devices in accordance with Ferrell have captured the turn-on transient in a microcomputer for visual comparison by a user to subsequent captured turn-on transient. Point-by-point comparison of the digitized turn-on transient by the microcomputer has proven to be very difficult and unreliable because the turn-on transients are not exactly the same for successive turn-ons of the same transmitter. Consequently, visual comparison has been used. The system in Ferrell is simply a computerized version of the radio fingerprinting of the late 40's and 50's and does not meet the needs of modern subscriber systems adequately. Visual comparison is not practical in large subscriber systems due to customer expectations of fast access and due to excessive cost of visual comparison. Additionally, other portions of a transmitter's signal contains information having much greater transmitter discrimination/identification value.
Cellular telephone systems encounter similar problems as those described above. Cellular telephones have an identification code including an electronic serial number (ESN) and a mobile identification number (MIN) assigned to each phone. When the cellular telephone user initiates a call, the telephone transmits the identification code assigned to the phone for billing and call authorization purposes.
Tampering with the phone to alter the MIN or ESN was supposed to result in an inoperable phone. However, fraudulent persons devised ways to obtain the identification code of an authorized cellular telephone and to input the numbers into an unauthorized cellular telephone. The unauthorized cellular telephone could be used for "free" while charges for the calls were billed to the authorized user. The unauthorized users also did not pay the basic monthly rates. Unfortunately, no other provision for user identification was typically built into the cellular telephone system.
Alternate methods proposed were unacceptable irritations to the users or were vulnerable to being defeated by fraudulent persons. For example, one proposed method includes a request for a user personal identification number (PIN) each time a call is made. The PIN could be transmitted on a different frequency. However, some users will be irritated by the PIN request and change to a different carrier who does not require PIN's.
PIN systems also presuppose that the authorized user desires to maintain the PIN number in secrecy. Even if PIN systems are made universal, they are still vulnerable to interception by fraudulent persons.
Other proposed methods include operator interaction with each caller for positive personal identification and per call requests for credit card numbers. However, these methods are economically unfeasible and are still subject to the same problems as the PIN method. Any system which requires identification data to be transmitted from the cellular telephone to the cell sites can be intercepted, copied, and used to gain unauthorized access.
In summary, any proposed solution must allow easy access while still providing protection to the operator of the subscriber service or telephone exchange.